The present invention relates to a fuel cell system, which circulates an anode exhaust gas discharged from a fuel cell so as to recycle it.
As a trend for restricting an amount of discharged carbon dioxide prevails, which is considered to cause earth warming, attention has recently concentrated on fuel cell electric vehicles (FCEV). An FCEV has an onboard fuel cell (FC), which generates power by electrochemical reaction between hydrogen (H2) and oxygen (O2) existing in air. Electricity generated by the fuel cell is supplied to a motor so as to produce driving power.
In a fuel cell system, hydrogen (anode gas), which is sent from a high-pressure hydrogen tank, is supplied after depressurization to an anode via a hydrogen supply line. On the other hand, air (cathode gas) pressurized by an electric compressor is supplied to a cathode via an air supply line. Hydrogen not consumed in the fuel cell (anode exhaust gas) is discharged via a line for discharging anode exhaust gas. Similarly, air having been subjected to reaction (cathode exhaust gas) is discharged via a line for discharging cathode exhaust gas.
The anode exhaust gas discharged from the fuel cell contains the hydrogen, which is not consumed as described above. A fuel cell system having a gas-liquid separator is known (See patent document 1), which separates reaction product water (hereinafter referred to as “product water”), which is contained in the form of water vapor, from an anode exhaust gas so as to recycle the unconsumed hydrogen. The gas-liquid separator, in which a supplied cooling liquid cools the anode exhaust gas so that the water vapor is condensed, discharges resulting liquid water via a drain line. On the other hand, the anode exhaust gas, from which the product water has been separated, is supplied to an ejector. The ejector carries out mixture of the anode exhaust gas and an anode gas sent from a high-pressure hydrogen tank, supplying the mixture to an anode of the fuel cell. In this way, recycling of hydrogen contained in the anode exhaust gas is implemented.    Patent document 1: Japanese Published Patent Application H8-321316 (paragraphs 0016-0017, FIG. 1)
It is known that because product water is contained at a high ratio in an anode exhaust gas discharged from a fuel cell, it sometimes remains in the anode exhaust gas due to incomplete separation when supplied to an ejector. If this happens, it is feared that liquid product water, which is generated by condensation of the product water remaining in the anode exhaust gas in a line connecting a gas-liquid separator and the ejector, may be sucked by the ejector. When an anode gas containing the liquid product water is supplied to the fuel cell, it may be that possible variation in a mass flow and pressure of the anode gas supplied to the fuel cell has an adverse effect on stability of power generation. This has been more frequently occurred when the line is arranged to be longer.
There has been another problem that when a fuel cell is left for a long period of time under low temperature conditions, below freezing point for example, with product water remaining in a drain line, which is separated by a gas-liquid separator, the product water is frozen to block the drain line, having an adverse effect on starting of the fuel cell.